1. Field
One embodiment of the invention relates to a telephone system using e.g. IP (Internet Protocol) technology.
2. Description of the Related Art
A well-known system communicates audio information through a best-effort type communication network such as the internet. This kind of system is called an IP telephone system or VoIP (Voice over IP) and applied to a local communication network such as a local telephone network as well as a wide-area telephone network. In the following description, the telephone terminal in such a kind of system is referred to as an IP terminal inclusive of a fixed telephone and a softphone.
Mostly, the IP telephone system has a function capable of authenticating an individual telephone. A typical example of such a terminal authentication function is a registration operation using an SIP (Session Initiation Protocol). This function enhances the independency of the IP telephone so that the IP telephone can be easily moved within the network. Namely, irrespectively of the installation position, the IP terminal having logged in the system can be employed by freely calling up the setting of a user himself and personal telephone directory.
In order to apply the SIP to the system, in many cases, a means of mounting the SIP is adopted in a dedicated server. However, since the SIP is a right protocol, mounting the SIP in an indispensable resource such as an exchange permits system cost to be reduced more greatly. Such an embodiment is widely adopted.
However, in the above-described related-art, the IP terminal can log in only the exchange to which the IP terminal is belonged. Namely, in a system where a plurality of exchanges each including a plurality of IP terminals are interconnected to one another through the network, the log-in/out function can be realized only within a small network closed for each exchange. Thus, it is inconvenient that the user cannot log in the system when the user moves from one small network to another small network.
It is disclosed by, for example, JP-A-2004-64225 that a management server for managing the relationship between node (exchange) information and address information is indispensable, so that cost of the system will be increased, since following procedures is required when the user logs in the system. Once the IP telephone is connected to the management server when the user logs in the system, the exchange of the other side is to be designated by inputting the node information, so that the user is forced to input the server information and the node information.